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THEORY AND OPERATION OF DIESEL ENGINES PART-1

Foreword

Bulletin No: 985616

This program has been developed to familiarize customers, sales and service personnel, and other iindividuals wi th Fundamental operating principles of diesel engines.

Combust ion -- Two stroke -- Four Stroke -- Air Induct ion Variables -- Engine performance Variables

Cummins Engine

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2. The diesel engine is a sothat harnesses the force of

obtain useful rotary motion

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3. Air alone is compressed cylinder and a charge of fu

into the cylinder when the top dead center. The ai r tehas risen suffici.ently at thifuel ignition.

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4. The resulting gas expanpiston downward at higt1v

piston accelerates rapidly ucompletes approximately hdownward stroke. From thedecelerates and comes to instantaneous dead stop atits stroke before continuing

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5. Diesel engines can be cvar1ety of groups with ehg

group having something in each other. The most basicclassification is by number reciprocating stroks taken tone comp)ete cycle. Modereci procatlng diesel engine

one cycle in either 2 pistonpiston strokes according toengine design.

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6. The 2-stroke cycle desigdeveloped by Sir Dugaid C

This design is applied to boignition and compression igbut in multi-cylinder enginecompression ignition type hsuccessful.

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7. In the 2-stroke cycle dieis sprayed into the hot com

when the piston is near topWhen the fueJ ignites, comto liberate the energy for thstroke whichfollows.

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8. When the piston is aboudown, exhaust valves begi

the cylinder pressure drops

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9. In another 10 to 30 degrcrankshaft rotation, the pis

enough to uncover fresh aiwhich have been cast into Clean air which has been pa rotary or centrifugal blowenter the cylinder.

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10. The air serves to blow texhaust gas upward and o

ports at the top of the cylinexhaust valves are complethis time due to precise timcamshaft action. Blowing tclean from the bottom inletthrough the exhaust ports

process called uniflow scav

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11. When the piston starts second and final stroke of t

exhaust valves close and afresh air is trapped inside tcylinder.This is the comprecombination of flywheel inepower stroke in another cythe power for th1s upward stroke.

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12. Near the top of the stroinjected and the cycle begiitself. Thus each cylinder indiesel engine produces powdownwad stroke of its pistoeach crankshaft revolution

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13. It should be noted that cycle diesel engine, the cathe same. speed as the craengine is of the V-type, usucamshafts are used and boengine speed. Since exhauopen, and fuel must be injeduring every crankshaft revcrank gear ratio to the cammust be one to one.

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14. In the 4-stroke cycle dieach cylinder produces powother downward stroke of tsequence of events to comcycle is as follows:

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15. The piston moves dowaction and in doing so creavacuum at the top of the cyrushes in through open pothe space left by the desce

This is the intake stroke.

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16. A few degrees, after thbegins; its upward movemevalves close and a quantitytrapped inside the cylinder

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17. As the piston continuesstroke, the space that confgets progressively smaller occupies about 1/16 of its ovolume. Compressing air inspace raises its temperatuand it is now ready to suppcombustion. This upward spiston is called the compre

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18. A few degrees before treaches the very top of its injected into the hot comprafter a very short delay perFuel continues to be injectepiston travels to top centerdegrees. there after, depenengine design, speed, and point, the cylinder pressuremaximum for that particula

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19. The furce from the burnfuel mixture acts on all surfcombustion chamber fairlythe force is able to move oand the piston moves dowvelocity. This is the 3rd stroand is called the expansionstroke.

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20. When the piston has co3/4 of its downward strokevalves open and cylinder pto drop toward atmospheriBecause cylinder pressure equalize, the piston can mout the rest of the exhaust words, there will be minimapressure for the piston to wits upstroke.

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21. The piston passes bottostarts its upstroke-forcing tout the exhaust ports.

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22. Before the piston reachits stroke, the intake valvesexact number of degrees bcenter depends on engine desirable for the intake valvthis time to foster maximum

This is especially so when at maximum rated RPM,anspeeds are high. Since botvalves are open at the pistposition between the exhastrokes, they are referred toverlapped.

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23. The exhaust valves areafter the piston passes tophigh exhaust pressure doeinto the intake manifold. Thstroke is the 4th stroke andthe cycle. The cycle then bitself.

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24. It should be remembere2 stroke and 4 stroke diesefuel is throttled. Engine spepower output are controlledthe amount of fuet sprayedcompressed air. It follows tand air are partners in the combuston process and boimportant.

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25. There should be an excair in the cylinder over and which is required for combideal and controlled condit

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26. Injecting more fuel by rmanifold pressure or by mefuel will not result in more f

the power stroke if there is to combine with it. Lack of results in power loss, smokexhaust temperature and rlife.

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27. How well a 4-stroke enpack the cylinder with cleacalled "volumetric efficienc

example, consider a cylind100 cubic inches of air at apressure when the piston isdead center with the enginonly 80 cubic inches of air cylinder at a certain engine

volumetric efficiency is 80%particular speed.

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28. In the 2-stroke cycle.enamount of clean, fresh air rtrapped in the cylinder is ac

importance than how much The amount retained will odirectly affect the amount obe burned and thus power has to do with what is calleefficiency"

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29. There are many variabdirectly affect how effieientreceives air. Engine design

do with it. That is, the smointernal air passages, the dnumber of bends in apassasize of the ports all have th

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30. Valve timing can also hpronounced effect on enginConsider a 4-stroke cycle e

that closes too soon. Part oexhaust gas can be pushedintake port during the valveperiod and clean incoming restricted.

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31. Also, intake valves andunduly heated when exhauescapes in this way. These

hot surfaces wiJ I heat incothe air will be less dense, wthat less oxygen will be avasupport combustion. Also cearly opening of the intaketo have the same effect.

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32. For these reasons, valvshould be perfomed as a remaintenance item. Taking

the high air velocities, the vmechanical actions that hamilliseconds, and the fact trocker arm clearance resuvalve timing, these regular valve adjustments are of pr

importance.

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33. Any restriction to air floair actually enters inlet pipiair filter will also reduce the

charge. For this reason, airshould be serviced at perioand quite frequently if the eoperating environment hasair-borne dirt.

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34. Another variable that waffect power output concerthe cylinder is sealed.

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35. Obviously, piston rings form a good seal with the cwill permit expanding gas t

down into the crankcase. Call of the fuel will not burn wcylinder for lack of oxygen gas and raw fuel vapor willexcessively hot.

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36. What is left will be undepressure due to increased and will tend to flow out the

under the same conditions pointed out earlier. Poor vatend to do the same thing beffective cylinder pressure

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37. There are many other vaffect or limit an engine's pSome of these are basic to

design. What can be done and what must be done metwo completely different thcompromises must necess

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38. Other variables includeunder which engines opera

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39. Still others involve the qquality of maintenance.

40 I l t t i

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Formated by -- Service

Cummins Diesel Sales and Service 35A/1/2, Erandawana,

40. In later programs, topicthese will be presented in idetail. Knowing, the fundam

operating principles of diesmake the learning of more operations much easier.

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